Rechargeable electric storage devices have been used. The electric storage devices can be repeatedly used by charging and discharging. Therefore, the electric storage devices are eco-friendly in comparison to non-rechargeable batteries and thus currently used in various fields including electric vehicles.
In a device including a plurality of electric storage devices, capacities of the electric storage devices may not be equal due to variations in initial capacity or different deterioration speeds. Some of the electric storage devices may reach a full-charge voltage earlier or later than the other electric storage devices. Therefore, some of the electric storage devices may not be fully charged. Further, in discharging, some of the electric storage devices may reach a discharging termination voltage and thus the discharging may be terminated earlier or later than the other electric storage devices. Therefore, power stored in some of the electric storage devices may not be fully used. Thus, if the capacities of the electric storage devices are not equal, those capacities cannot be fully utilized. A technology for discharging the secondary batteries with uneven capacities by a discharging circuit has been known. The discharging circuit including a resistor is provided for equalizing the capacities of the electric storage devices. According to the technology, remaining capacities of the electric storage devices are determined based on information on voltages of the secondary batteries. The information is obtained while no currents flow through the electric storage devices. The electric storage devices are discharged based on differences in capacity to equalize the capacities.
Recently, an olivine-type lithium-ion iron secondary battery (hereinafter referred to as an olivine-type iron battery) draws attention as a secondary battery for electric vehicles. The olivine-type iron battery is a kind of lithium-ion batteries and has a positive electrode made of olivine-type iron phosphate and a negative electrode made of, for example, a graphite-based material. Since the olivine-type iron battery need not use a cobalt-based material as its electrode, it is more inexpensive and safer than a secondary battery that uses a cobalt-based electrode.
The olivine-type iron battery is known to have a region (hereinafter referred to as a variation region) where a voltage sharply increases with respect to an increase in a remaining capacity. If the olivine-type iron battery having a negative electrode made of a graphite-based material, the battery has a variation region in which an SOC indicating the remaining capacity of a secondary battery is lower than 10% and another variation region in which the SOC is equal to or higher than 90%. Since the battery has the variation region where the SOC is relatively high or low, even if equalization of the capacities of electric storage devices are performed based on voltage information of the electric storage devices in the variation region, the SOCs of the electric storage devices reach about 100% or about 0% before the capacities of the electric storage devices are equalized. As a result, the charging or the discharging of the electric storage devices may be terminated. If the charging or the discharging of the electric storage devices is terminated, the equalization of the capacities of the electric storage devices may also be terminated. Therefore, the capacities may not be sufficiently equalized. Accordingly, a technology for equalizing the capacities of the electric storage devices using a region other than the variation regions is needed.
The olivine-type iron battery is known to have a plateau region in combination with the negative electrode. For example, when the negative electrode is made of a graphite-based material, the olivine-type iron battery may have a plateau region including an SOC value, which indicates the remaining capacity of secondary batteries, of 10% to 90%. In this context, the plateau region refers to a region where the voltage of a secondary battery stays substantially constant irrespective of changes in SOC of the secondary battery. In the electric storage devices such as the secondary battery having a plateau region, the capacities of the electric storage devices may not be easily estimated based on voltage information of the electric storage devices obtained in the plateau region and, therefore, the capacities of the electric storage devices may not be properly equalized.